Despite the ever-present joke about avoiding your in-laws and "crazy Cousin Steve" at all costs, bringing family together for the holidays, no matter how painful, is still something to celebrate.

Daddy's Home 2, the sequel to 2015's smash hit starring Mark Wahlberg and Will Ferrell, puts a delightful spin on the holiday genre, revealing the harmony—and chaos—that ensues when two families unite for the "perfect" Christmas.

Co-dads Dusty and Brad are getting along much better this time around, but things take a sharp turn when Dusty's old school dad Kurt (Mel Gibson) and Brad's ultra-nurturing father Don (John Lithgow) show up.

TheFIX caught up with Gibson and Lithgow in Los Angeles to find out how the elder dads bring new challenges for Dusty and Brad.

Image: Paramount Pictures

"I think the Kurt character is more of a catalyst," said Gibson, 61. "He brings his own disfunction and wants to inflict it on everybody else. But there's really a sort of 'want' in that for him, to poke at the wound. Sort of, 'maybe something will happen' even though he's in denial."

Added Lithgow, "And Don is blithely oblivious. He doesn't even know there's bad vibes. The interesting thing about the character, though, is that he has a secret, and all of his good cheer is covering up something he dreads revealing."

Image: Paramount Pictures

Daddy's Home 2 centers on the intricate dynamics of fatherhood, navigating everything from the bitter battle for control over the thermostat to decades-old grievances between fathers and sons. Both Gibson and Lithgow, 72, had an easy time working around these themes, being fathers themselves.

"For one thing, everybody was a father, all four of us guys. We all had access to that. And a lot of us have had the experience of needing to blend the family," said Lithgow.

Image: Paramount Pictures

"I've got more children than any of them," joked Gibson, who welcomed his ninth child, a son named Lars Gerard Gibson, with girlfriend Rosalind Ross in January.

"It's a wonderful thing. My goodness, we're getting into double-digit generations," he said, adding that the "family atmosphere" on set made him feel right at home.

Image: Paramount Pictures

"It's always good to know the real thing so you can make something funny out of it," said Lithgow.

And when you team Lithgow and Ferrell, two of Hollywood's greatest comedic performers, "anything can happen," revealed Lithgow, who full-on kissed Ferrell in a so-good-it's-cringeworthy moment on-screen.

In the scene, the affectionate Brad and Don lock lips after reuniting at the airport. You know, as one does.

"It was scripted," recalled Lithgow. "And even reading it in the script I thought, 'Wow, this is going to define this whole movie.'"

As for how they prepared for the scene, Gibson playfully jumped in: "There's a lot of mouthwash."

Added Lithgow: "Will is completely game for anything. He's done many years of Saturday Night Live. I've done many years on 3rd Rock from the Sun. We knew perfectly well this was a gangbusters comedy moment, and we just went for it without the slightest self-consciousness."

And if you're wondering if Ferrell is a good kisser?

"Not particularly," Lithgow admitted, chuckling softly, and using his fists to demonstrate. "We just sort of mashed our faces together like medicine balls."